Re: [BUG] Early OOM and kernel NULL pointer dereference in 4.19.69

[Thomas Lindroth <thomas.lindroth@gmail.com>]

On 9/2/19 9:16 AM, Michal Hocko wrote:
> On Sun 01-09-19 22:43:05, Thomas Lindroth wrote:
>> After upgrading to the 4.19 series I've started getting problems with
>> early OOM.
> 
> What is the kenrel you have updated from? Would it be possible to try
> the current Linus' tree?

I did some more testing and it turns out this is not a regression after all.

I followed up on my hunch and monitored memory.kmem.max_usage_in_bytes while
running cgexec -g memory:12G bash -c 'find / -xdev -type f -print0 | \
         xargs -0 -n 1 -P 8 stat > /dev/null'

Just as memory.kmem.max_usage_in_bytes = memory.kmem.limit_in_bytes the OOM
killer kicked in and killed my X server.

Using the find|stat approach it was easy to test the problem in a testing VM.
I was able to reproduce the problem in all these kernels:
   4.9.0
   4.14.0
   4.14.115
   4.19.0
   5.2.11

5.3-rc6 didn't build in the VM. The build environment is too old probably.

I was curious why I initially couldn't reproduce the problem in 4.14 by
building chromium. I was again able to successfully build chromium using
4.14.115. Turns out memory.kmem.max_usage_in_bytes was 1015689216 after
building and my limit is set to 1073741824. I guess some unrelated change in
memory management raised that slightly for 4.19 triggering the problem.

If you want to reproduce for yourself here are the steps:
1. build any kernel above 4.9 using something like my .config
2. setup a v1 memory cgroup with memory.kmem.limit_in_bytes lower than
    memory.limit_in_bytes. I used 100M in my testing VM.
3. Run "find / -xdev -type f -print0 | xargs -0 -n 1 -P 8 stat > /dev/null"
    in the cgroup.
4. Assuming there is enough inodes on the rootfs the global OOM killer
    should kick in when memory.kmem.max_usage_in_bytes =
    memory.kmem.limit_in_bytes and kill something outside the cgroup.